The present invention relates to an apparatus as defined in the preamble of claim 1, and a method as defined in the preamble of claim 7, i.e. a method and an apparatus for charging a piezoelectric element.
The present piezoelectric elements being considered in more detail are, in particular but not exclusively, piezoelectric elements used as actuators. Piezoelectric elements can be used for such purposes because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
A The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitative elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are described below and are incorporated herein by reference in their entirety, disclose piezoelectric elements with double acting, double seat valves for controlling injection needles in a fuel injection system.
In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element. If the valve plug of the control valve is located in one of the two seats of the double seat valve, the nozzle needle remains or becomes closed. If the valve plug is in an intermediate position between the seats, then the nozzle needle remains or becomes open. A goal is to achieve a desired fuel injection volume with high accuracy, especially at small injection volumes, for example during pre-injection.
In the example of a double acting control valve, the piezoelectric element is to be expanded or contracted by the effect of an activation voltage applied to the piezoelectric element, so that a corresponding controlled valve plug is positioned midway between the two seats of the double acting control valve to position the corresponding injection needle for maximum fuel flow during a set time period. It has proven to be difficult to determine and apply an activation voltage suitable for all injection elements and the whole lifetime of the injection system with sufficient precision such that, for example, a corresponding valve plug is accurately positioned for maximum fuel flow.
It is therefore an object of the present invention to develop the apparatus as defined in the preamble of claim 1 and the method as defined in the preamble of claim 7 in such a way that an activation voltage level for a piezoelectric element is set with sufficient precision to, for example, accurately position a valve plug for maximum fuel flow. The particular piezoelectric element can be one of several piezoelectric elements used as actuators in a system such as, for example, a fuel injection system.
This object is achieved, according to the present invention, by way of the features claimed in the characterizing portion of claim 1 (apparatus) and in the characterizing portion of claim 7 (method).
These provide for:
an activation voltage value for charging a piezoelectric element to be controlled online by an optimization unit which adjusts the value of the activation voltage as a function of operating characteristics of the particular piezoelectric element and the hydraulic components (characterizing portion of claim 1); and for
a definition to be made, prior to charging, as to a value for an activation voltage for charging a piezoelectric element, as a function of operating characteristics of the particular piezoelectric element and the hydraulic components (characterizing portion of claim 7).
The amount of expansion or contraction of piezoelectric elements is influenced by operating characteristics of each particular piezoelectric element, and can vary from sample-to-sample and/or with the ages of the piezoelectric elements. Thus, it has been determined, according to the present invention, that the amount of displacement of a particular actuator implemented as a piezoelectric element, in response to application of a particular voltage can vary as a function of, for example, the operating characteristics of the particular piezoelectric element, and/or the age of the actuator. The result is that actuators behave differently when charged to the same voltage, and their operation can vary over time.
Accordingly, in this example, the activation voltage level for a piezoelectric element, suitable for displacement of the element sufficient to move the injection needle to an optimum position for maximum fuel flow, in the example of a double acting control valve, is influenced by operating characteristics, and changes in the operating characteristics with age, of the particular piezoelectric element. In terms of fuel injection and injection profile, this generally infers a deviation from optimum system operation. Furthermore, sample-to-sample deviation and aging effects of the hydraulic components themselves influence the behavior of the whole injection system, too. Prior to the present invention, these effects could be overcome only by designing injection equipment so robustly that variations due to particular operating characteristics and age of actuators have little or no repercussions in system operation.
Given activation voltage levels set as a function of operating characteristics of particular ones of piezoelectric elements to be charged, control valves can be controlled with sufficient accuracy independently of sample-to-sample variations of operating characteristics of the actuators, or changes in operating characteristics with age. The activation voltage applied to a piezoelectric element at any particular time will be appropriate relative to the operating characteristics of the particular piezoelectric element at the time of application of the voltage. In this manner, a desired injection volume can be achieved with sufficient accuracy even if the injection volume is small or the injection profile complex.
Advantageous developments of the present invention are evident from the dependent claims, the description below, and the figures.